MECHANISM OF THE TRANSFORMATION OF PRESSURE FLUCTUATION PRODUCED BY KARMAN VORTEX SHEDDING INTO NOISE

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It is theoretically and experimentally investigated that the generated noise is closely related with the characteristics of the velocity fluctuation in the near wakes of a blade,NACA 0008 or NACA 0015 placed in a uniform oncoming flow,and in the near wake of a rotating flat plate. The results are summarized as follows:(1)In the case of a thicker blade,NACA 0015,the discrete frequency noise is generated if the attack angle theta is less than about 5 degree. The flow on the pressure side, separates in this case just upstream of the trailing edge of the blade and the Karman vortex street generates,which is the cause to generate the discrete frequency noise. (2)In the case of a thinner blade,NACA 0008,the DFN does not occur if theta=0,because there is no location where adverse pressure gradient is intense. However if there is some attack angle,even if it is small,the flow separates near the leading edge of the blade due to its relatively sharp geometry. Then discrete frequency noise generates. (3)In the case of NACA 0008 blade periodic velocity fluctuation is actually generated relatively far from the blade trailing edge,the DFN,however,does not occur. The reason of this is the flow attached up to the blade trailing edge. This fact shows clearly that the necessary condition of the generation of DFN is the existence of a large dead flow region in the near wake. (4)Although the oncoming flow is sheared in the span direction as in the case of the rotating flow field,DFN occurs. This means the mechanism of the generation of DFN is fundamentally the same that in the uniform main flow. (5)The broad band nature of the spectrum of noise generated from the rotating flat plate can be theoretically explained by the physical model in which the tonal noises are generated locally by the periodic velocity fluctuations due to each Karman vortex shedding and they are integrated in the blade span direction.